Recent studies have characterized the attributes of a class of bacterial and viral proteins, termed superantigens. Compared to conventional antigens, this type of antigen binds and stimulates 1,000-10,000 times larger subsets of lymphocytes. To date, only superantigens that bind specifically to sites on T cell receptor VBeta regions have been described. We now have data which suggests that Staphylococcal protein A (SPA) has a comparable interaction with Beta cell Ig receptors, due to binding to a site that is structurally separate from the Fc region of IgG. Initial studies have demonstrated that SPA is bound by 10-50% of polyclonal IgM, IgA, and IgG Fab'2 from healthy adults, and an equivalent proportion of monoclonal IgM, because binding to this "alternative binding site" of SPA is restricted to VH3 Ig. Significantly, at least four different germline genes from the VH3 family can encode for SPA binding, while binding is independent of VL family usage. Furthermore, our initial DNA sequence analysis suggests that this SPA binding activity correlates with conserved motifs in the VH3 Fab framework regions, but not with hypervariable region sequences. In addition, by microfluorimetry the alternative binding site of SPA is bound by 20-35% of Ig bearing Beta cells in splenic and tonsillar lymphoid tissue. To investigate the properties of this proposed model Beta cell superantigen, we propose to: (i) use a newly developed expression vector to further structurally characterize the sites on human Beta cells that interact with the alternative binding site of SPA at the molecular level, ii) assess the functional capacity of this proposed Beta cell superantigen to induce in vitro Beta cell activation, and iii) evaluate the capacity for SPA to influence antibody repertoire formation after in vivo immunization. These studies are essential to determine whether superantigens may contribute to the physiologic acquisition of the Beta cell repertoire. Furthermore, these investigations will likely suggest new approaches to the augmentation of anti-microbial immune responses that are commonly encoded by genes from the VH3 family.